Multi-antenna wireless transmission technique, or Multiple In Multiple Out (MIMO), can achieve spatial multiplex gain and spatial diversity gain by deploying a plurality of antennas at both the transmitter and the receiver and utilizing the spatial resources in wireless transmission. Researches on information theory have shown that the capacity of a MIMO system grows linearly with the minimum of the number of transmitting antennas and the number of receiving antennas.
FIG. 1 shows a schematic diagram of a MIMO system. As shown in FIG. 1, a plurality of antennas at the transmitter and a plurality of antennas at each of the receivers constitute a multi-antenna wireless channel containing spatial domain information. Further, Orthogonal Frequency Division Multiplexing (OFDM) technique has a strong anti-fading capability and high frequency utilization and is thus suitable for high speed data transmission in a multi-path and fading environment. The MIMO-OFDM technique, in which MIMO and OFDM are combined, has become a core technique for a new generation of mobile communication.
For instance, the 3rd Generation Partnership Project (3GPP) organization is an international organization in mobile communication field which plays an important role in standardization of 3G cellular communication technologies. Since the second half of the year 2004, the 3GPP organization has initiated a so-called Long Term Evolution (LTE) project for designing Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN). The MIMO-OFDM technique is employed in the downlink of the LTE system. In a conference held in Shenzhen, China in April 2008, the 3GPP organization started a discussion on the standardization of 4G cellular communication systems (currently referred to as LTE-A systems). Again, the MIMO-OFDM technique becomes a key technique for air interface in the LTE-A system.
In the LTE-A system, Carrier Aggregation (CA) is a new concept. FIG. 2 illustrates the CA concept in which a base station is provided with a plurality of downlink carriers and a plurality of uplink carriers. A number of carriers can be virtually combined into one carrier, which is referred to as carrier aggregation. The LTE-A system can support continuous CA as well as intra-band or inter-band non-continuous CA, with a maximum aggregated bandwidth of 100 MHz. In order to ensure effective utilization of the carriers at the initial stage of the commercial deployment of the LTE-A system, i.e., to ensure that LTE UEs can access the LTE-A system, each carrier should be configured to be backward compatible with the LTE system. However, it is also possible to design a carrier dedicated to the LTE-A system.
In the LTE-A system, a concept known as Coordinated Multi-Point (CoMP) transmission gets extensive attention and support. The core idea of CoMP is to solve the problem of downlink inter-cell interference by coordination between a plurality of base stations (cells), thereby providing improved data transmission rate and user experience for a UE located at the edge of a cell.
At the research stage of the LTE-A system, one of the major research subjects relates to design of control channel, including feedback of downlink CSI from a UE to a BS.
There are two feedback channels for downlink CSI feedback, a Physical Uplink Control CHannel (PUCCH) and a Physical Uplink Shared CHannel (PUSCH). In general, the PUCCH is configured for transmission of synchronized, basic CSI with low payload; while PUSCH is configured for transmission of bursty, extended CSI with high payload. For the PUCCH, a complete CSI is composed of different feedback contents which are transmitted in different sub-frames. For the PUSCH, on the other hand, a complete CSI is transmitted within one sub-frame. Such design principles remain applicable in the LET-A system.
The feedback contents can be divided into three categories: Channel Quality Index (CQI), Pre-coding Matrix Index (PMI) and Rand Index (RI), all of which are bit quantized feedbacks. In the LTE-A system, these three categories of contents are still the primary feedback contents.
In the LTE-A system, the following eight types of MIMO transmission approaches for downlink data are defined:
1) Single antenna transmission. This is used for signal transmission at a single antenna BS. This approach is a special instance of MIMO system and can only transmit a single layer of data.
2) Transmission diversity. In a MIMO system, diversity effects of time and/or frequency can be utilized to transmit signals, so as to improve the reception quality of the signals. This approach can only transmit a single layer of data.
3) Open-loop space division multiplexing. This is a space division multiplexing without the need for PMI feedback from UE.
4) Closed-loop space division multiplexing. This is a space division multiplexing in which PMI feedback from UE is required.
5) Multi-user MIMO. There are multiple UEs simultaneously participating in the downlink communication of the MIMO system.
6) Closed-loop single layer pre-coding. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is required.
7) Beam forming transmission. The beam forming technique is employed in the MIMO system. A dedicated reference signal is used for data demodulation at UE. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is not required.
8) Two-layer beam forming transmission. The UE can be configured to feed back PMI and RI, or not to feed back PMI and RI.
In the LTE-A system, the above eight types of transmission approaches may be retained and/or canceled, and/or a new transmission approach, dynamic MIMO switching, can be added, by which the BS can dynamically adjust the MIMO mode in which the UE operates.
In the LTE-A system, if a UE is configured to operate in the CA status, the transmission approach on each carrier for the UE can be configured to be one of the above transmission approaches via upper layer signaling (in a semi-static manner). On the other hand, if a UE is configured to operate in the CoMP status, the transmission approach on a carrier for the UE may be configured to be the above transmission approach 4), 5) or the dynamic MIMO switching approach.
In order to support the above MIMO transmission approaches, a variety of CSI feedback modes are defined in the LTE system, all of which are inherited by the LTE-A system. Each MIMO transmission approach corresponds to a number of CSI feedback modes, as detailed in the following.
There are four CSI feedback modes for the PUCCH, Mode 1-0, Mode 1-1, Mode 2-0 and Mode 2-1. These modes are combination of four types of feedbacks, including:
1) Type 1: one preferred sub-band location in a Band Part (BP, which is a subset of the Set S and has its size dependent on the size of the Set S) and a CQI for the sub-band. The respective overheads are L bits for the sub-band location, 4 bits for the CQI of the first codeword and 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword.
2) Type 2: broadband CQI and PMI. The respective overheads are 4 bits for the CQI of the first codeword, 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword and 1, 2 or 4 bits for PMI depending on the antenna configuration at BS.
3) Type 3: RI. The overhead for RI is 1 bit for two antennas, or 2 bits for four antennas, depending on the antenna configuration at BS.
4) Type 4: broadband CQI. The overhead is constantly 4 bits.
All the frequency areas corresponding to the CSI feedback are referred to as Set S. In the LTE system where there are only single-carrier situations, the Set S is defined as equal to the carrier bandwidth of the system. In the LTE-A system where there are additionally multi-carrier situations, the Set S can be defined as equal to the bandwidth of one single carrier or equal to the summed bandwidth of multiple carriers.
The UE feeds back different information to the BS in correspondence with the above different types.
The Mode 1-0 is a combination of Type 3 and Type 4. That is, the feedbacks of Type 3 and Type 4 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-0, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back.
The Mode 1-1 is a combination of Type 3 and Type 2. That is, the feedbacks of Type 3 and Type 2 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-1, the broadband PMI of the Set S, the broadband CQIs for the individual codewords and possibly the RI information are fed back.
The Mode 2-0 is a combination of Type 3, Type 4 and Type 1. That is, the feedbacks of Type 3, Type 4 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-0, the broadband CQI of the first codeword in the Set S, possibly the RI information as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.
The Mode 2-1 is a combination of Type 3, Type 2 and Type 1. That is, the feedbacks of Type 3, Type 2 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-1, the broadband PMI of the Set S, the broadband CQIs for the individual codewords and possibly the RI information, as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.
There are thus the following correspondence between the MIMO transmission approaches and the CSI feedback modes:
MIMO transmission approach 1): Mode 1-0 and Mode 2-0;
MIMO transmission approach 2): Mode 1-0 and Mode 2-0;
MIMO transmission approach 3): Mode 1-0 and Mode 2-0;
MIMO transmission approach 4): Mode 1-1 and Mode 2-1;
MIMO transmission approach 5): Mode 1-1 and Mode 2-1;
MIMO transmission approach 6): Mode 1-1 and Mode 2-1;
MIMO transmission approach 7): Mode 1-0 and Mode 2-0;
MIMO transmission approach 8): Mode 1-1 and Mode 2-1, with PMI/RI feedback from UE; and
MIMO transmission approach 8): Mode 1-0 and Mode 2-0, without PMI/RI feedback from UE.
In the CoMP status, the feedback modes for a UE are consistent with those corresponding to the above transmission approaches 4) and 5). That is, the feedback modes for a UE operating in the CoMP status incorporate Mode 1-1 and Mode 2-1. However, the Mode 1-1 and Mode 2-1 in the LTE-A system are optimized for a scenario where a BS is equipped with 8 transmission antennas. That is, a PMI is collectively determined from two channel pre-coding matrix indices, W1 and W2, where W1 represents broadband/long-term channel characteristics and W2 represents sub-band/short-term channel characteristics. For transmission of W1 and W2 over PUCCH, Mode 1-1 can be sub-divided into two sub-modes: Mode 1-1A and Mode 1-1B. In Mode 1-1A, the RI and the down-sampled W1 are jointly coded and then fed back as Type 3 and W2 is fed back as Type 2. In Mode 1-1B, the down-sampled W1 and the down-sampled W2 are jointly coded and fed back as Type 2. Accordingly, Mode 2-1 can also be sub-divided into two sub-modes: Mode 2-1A and Mode 2-1B. In both Mode 2-1A and Mode 2-1B, the RI and 1-bit information known as Pre-coding Type Indicator (PTI) are jointly coded and fed back as Type 3. The PTI having a value of 0 and 1 indicates that the corresponding sub-mode is Mode 2-1A and Mode 2-1B, respectively. In Mode 2-1A, W1 is fed back as Type 2 and the broadband W2 is fed back as Type 1, while in Mode 2-1B, the broadband W2 is fed back as Type 2 and the sub-band W2 is fed back as Type 1. It is to be noted that the Type 1 as used herein differs from the Type 1 defined in LTE Release 8 in that not only the CQI, but also the corresponding PMI, need to be fed back. In this regard, reference can be made to 3GPP R1-106514, “Way Forward on further details about PUCCH”.
In the CoMP status, a UE may need to report, for a set of cells, the channel state/statistical information for the links between the UE and BSs of the respective cells. This set of cells is referred to as CoMP measurement set. The cells for which the information is actually fed back by the UE can be a subset of the measurement set, referred to as report set. The measurement set can be the same as the CoMP coordination set which contains cells (BSs) participating in the Physical Downlink Shared Channel transmission for the UE, directly or indirectly. The coordination set may be or may be not transparent to the UE.
For a UE configured in the CoMP status, the feedback is mainly carried out separately in each cell and is transmitted over the uplink resources of the serving cell.
On the other hand, there are five CSI feedback modes for the PUSCH, Mode 1-2, Mode 3-0, Mode 3-1, Mode 2-0 and Mode 2-2.
In the Mode 1-2, the PMIs of the individual sub-bands in the Set S, the broadband CQIs of the individual sub-bands in the Set S and possibly the RI information are fed back.
In the Mode 3-0, the CQI for the first codeword of each sub-band in the Set S, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back. Herein, the sub-band CQIs are differentially coded with respect to the broadband CQI, so as to reduce feedback overhead.
In the Mode 3-1, the CQIs for the individual codewords of each sub-band in the Set S, the broadband CQIs of the individual codewords in the Set S, the broadband PMI of the Set S and possibly the RI information are fed back. Herein, the sub-band CQIs are differentially coded with respect to the broadband CQIs, so as to reduce feedback overhead.
In the Mode 2-0, the locations of the preferred M sub-bands in the Set S, the broadband CQI for the first codeword in each of the M sub-bands, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back.
In the Mode 2-2, the locations of the preferred M sub-bands in the Set S, the broadband PMIs for the M sub-bands, the broadband CQIs for the individual codewords in each of the M sub-bands, the broadband PMI of the Set S, the broadband CQIs of the individual codewords in the Set S and possibly the RI information are fed back.
There are thus the following correspondence between the MIMO transmission approaches and the CSI feedback modes:
MIMO transmission approach 1): Mode 2-0 and Mode 2-0;
MIMO transmission approach 2): Mode 2-0 and Mode 3-0;
MIMO transmission approach 3): Mode 2-0 and Mode 3-0;
MIMO transmission approach 4): Mode 1-2, Mode 2-2 and Mode 3-1;
MIMO transmission approach 5): Mode 3-1;
MIMO transmission approach 6): Mode 1-2, Mode 2-2 and Mode 3-1;
MIMO transmission approach 7): Mode 2-0 and Mode 3-0;
MIMO transmission approach 8): Mode 1-2, Mode 2-2 and Mode 3-1, with PMI/RI feedback from UE; and
MIMO transmission approach 8): Mode 2-0 and Mode 3-0, without PMI/RI feedback from UE.
In the LTE system, the periodic CSI feedback occupies the feedback resources on PUCCH. Its resource allocation involves frequency domain resource allocation, which is determined from a parameter nPUCCH(2) (cf. TR36.211 V9.1.0, “Physical channel and modulation”), and time domain resource allocation, which is determined from the feedback periods and sub-frame offsets of the respective feedback types (cf. TR36.213 V9.1.0, “Physical layer procedures”). In the LTE-A and its subsequent systems, there are two approaches for increasing the payload of uplink periodic feedback while maintaining the uplink single-carrier characteristic: one is to feed back several sets of feedback contents cyclically in time and the other is to increase the payload of one feedback by means of periodic PUSCH feedback.
There are currently few references available for the downlink multi-antenna multi-carrier multi-cell CSI feedback in the LTE-A system, as this has not been discussed in the standardization process. However, the multi-carrier CSI feedback is currently under discussion in the standardization process. Approaches which are now extensively supported include (cf. 3GPP R1-106525, “Way Forward on Periodic CQI/PMI/RI in CA”):
1) The high layer configuration parameters of periodic CQI/PMI/RI feedback can be configured for each downlink carrier individually according to the LTE Rel-8 specification.
2) If the PUCCH+PUSCH concurrent transmission characteristic is not enabled, periodic CQI/PMI/RI for only one downlink carrier is fed back in a sub-frame. The downlink carrier is selected based on a defined priority, which applies to both scenarios with and without PUSCH. The periodic CQI/PMI/RI feedbacks for other downlink carriers will be discarded.
3) The collision between different feedback types on the selected downlink carrier is solved according to the LTE Rel-8 specification.